Vacuum technology is widely utilized in various fields of semiconductor production, solar cells, aircraft, automobiles and optoelectronics. For this, widely known are mechanical vacuum pumps such as piston vacuum pumps, rotary vacuum pumps, etc.; and high-vacuum pumps such as oil-rotation vacuum pumps, oil-diffusion vacuum pumps, etc. Various synthetic oil based and mineral oil based vacuum pump oils are used for lubricating the sliding parts of those vacuum pumps, for ensuring high vacuum degrees and for prolonging the pump life.
With the recent developments in their application field, vacuum pumps are required to have high thermal stability and to attain high vacuum degrees. For this, various improvements have heretofore been made in vacuum pump oils. In addition, in the application field of vacuum technology, it is desired to shorten the time from the starting of vacuum pumps to the steady-state driving thereof in order to increase the producibility in the art. However, the low-temperature starting capabilities of conventional vacuum oils are often poor. Therefore, when they are used in winter or in cold districts, they take a long period of time before they reach the steady-state driving condition. For these reasons, there are various problems with conventional vacuum pump oils in that the producibility of intended products is low and that products of stable quality could not be obtained.